The invention relates to an electrohydraulic actuator. In particular, the invention relates to an actuator for actuating a hydraulic system on board a motor vehicle.
A system on board a motor vehicle, e.g. a clutch or brake system, can be actuated hydraulically. In a conventional motor vehicle driven by means of an internal combustion engine, the hydraulic pressure can be supplied by a pump coupled to the internal combustion engine. Sometimes, it is desirable also to be able to supply the hydraulic pressure independently of the internal combustion engine. In an electric or hybrid vehicle, it is not possible to assume in all circumstances that there will be torque available from a motor of the motor vehicle to drive the pump or that there will be a vacuum from the intake manifold to support a vacuum brake booster, for example.
An electrohydraulic actuator comprises an electric drive and a hydraulic pump. The drive acts on the pump, which supplies a hydraulic pressure of a fluid for actuating the clutch or the brake, for example. In one embodiment, an axial pump is used, comprising a hydraulic cylinder with a piston which can be actuated axially by means of a spindle drive. The spindle drive comprises a spindle and a nut, wherein the spindle is secured against rotation by means of a rotation-prevention mechanism and the nut can be rotated by means of the electric motor. In order to transmit the rotary motion of the electric motor to the nut and, at the same time, to reduce its speed, a worm gear can be used, for example.
Owing to the unavoidable friction between the nut and the spindle and the gearing forces on the nut, there is not only a torque but also a tilting force acting on the nut when the electric motor is running. As a result, a radial force can be transmitted to the piston, pressing the piston against the cylinder wall. The piston or a piston seal can thereby be subjected to increased wear. Particularly in the case of a system relevant to safety, such as an electrohydraulic actuator for a brake system, it can be crucial to ensure the full performance of the actuator, even over extended periods of operation.